THE PRINCIPLE OBJECTIVE OF THIS STUDY IS THE DETERMINATION OF FUNCTIONAL ASPECTS OF ACETYLCHOLINE (Ach)-ACTIVATED NICOTINIC CHANNELS OF VERTEBRATE SYMPATHETIC GANGLIONIC NEURONS. This will be accomplished through analysis of the mechanisms by which two different classes of cholinergic blocking agents reduce nicotinic cholinergic responses on ganglionic neurons. One class, identical to simple open channel blockers, appears to plug ACh channels and prevent channels from closing. I have identified a second group of compounds that on frog and lobster neuromuscular ACh channels appear to plug channels and, additionally, become trapped in the channel when it closes. The latter group may permeate the channel and have access to the blocking site from each end of the channel. Through the use of voltage-jump induced current relaxations in cultured sympathetic ganglionic neurons and the use of patch-clamp recordings of ACh-opened channels, the rates of blocking reactions, the voltage-dependence of the rates, and the site of blocker action for the two classes of drugs will be examined. The unusual properties of the blockers that become trapped in channels will allow the determination of blocking rates of the drugs from either end of the channel. This unique blocking mechanism when compared to the action of simple channel blockers will 1. provide new insight into the understanding of ionic channel blocking reactions, 2. provide the first comprehensive study of structural aspects of the ganglionic nicotinic ACh channel, and 3. for the first time, provide some molecular basis for understanding differences between neuromuscular and ganglionic nicotinic ACh channels. The drugs that become trapped in closed channels are of clinical interest since they are best-known as long-lasting anti-hypertensive agents. The proposed mechanism of action of these agents involves a use-dependent recovery from blockade which may account for their prolonged action. This study will clearly define two distinct molecular mechanisms of drug action. The ability to design drugs suited for particular therapeutic functions with minimal side-effects rests on an understanding of the molecular mechanisms of different classes of drugs.